Starch is a polymeric carbohydrate of very high molecular weight. Its monomeric units, termed anhydroglucose units, are derived from dextrose, and the complete hydrolysis of starch yields dextrose. In the United States, dextrose is manufactured from corn starch; in Europe from corn starch and potato starch; and in Japan from corn starch and white sweet potato starch.
Until 1960, dextrose was prepared from starch by acid hydrolysis. The method of preparation involved heating starch with hydrochloric or sulfuric acid at temperatures of 120.degree.-145.degree. C., then neutralizing the hydrolysis mixture with sodium carbonate, clarifying, and crystallizing the dextrose. Unfortunately, the yield of dextrose is lowered by the formation of relatively large amounts of reversion products, i.e., products which are formed by the recombination of dextrose molecules. Also, because of the high temperature and low pH of the hydrolysis reaction, some of the dextrose produced is converted to hydroxymethylfurfural, levulinic acid and color bodies. The formation of such degradation products is irreversible and, to the extent they are formed, the yield of desired dextrose is, of course, adversely affected. Still further, the use of hydrochloric acid or in some instances, sulfuric acid, and the subsequent neutralization of this acid with alkali results in the formation of inorganic salts which interfere with crystallization of the final dextrose product.
Later, hydrolysis of starch to dextrose was accomplished by means of enzymes. The principal enzyme used for this purpose was, and continues to be, glucoamylase. This enzyme effectively hydrolyzes the starch by cleaving one molecule of dextrose at a time from the starch molecule. As a practical matter, however, it is necessary first to reduce the molecular weight of the starch by partial hydrolysis before subjecting it to the action of glucoamylase. This process, called thinning, may be accomplished either by means of acid or enzyme. The starch is thinned to a dextrose equivalent (D.E.) of about 10-20, then treated with glucoamylase. This two-stage process is referred to as an acid-enzyme process or an enzyme-enzyme process, depending upon the nature of the thinning step employed.
In the acid-enzyme process, starch is liquefied and hydrolyzed in an aqueous suspension containing 20 to 40 percent starch and an acid, such as hydrochloric acid. The suspension is then heated to a high temperature, i.e., a temperature between about 70.degree. C. and about 160.degree. C. and at a pH between about 1 and 4.5 to liquefy and partially hydrolyze the starch. Typical acid-enzyme processes are disclosed in U.S. Pat. Nos. 2,305,168; 2,531,999; 2,893,921; 3,021,944 and 3,042,584.
In the enzyme-enzyme process, starch is liquefied and partially hydrolyzed in an aqueous suspension containing 20 to 40 percent starch and a liquefying enzyme, such as bacterial .alpha.-amylase enzyme at a temperature of from about 85.degree. C. to about 105.degree. C. The dextrose equivalent of the liquefied and partially hydrolyzed starch is generally less than about 20 and preferably less than about 10. The mixture is then subjected to a temperature above about 95.degree. C. and preferably between 110.degree. C. and 150.degree. C. to insure complete starch solution. The starch hydrolyzate is then cooled to a temperature of less than 95.degree. C. and subjected to further treatment with bacterial .alpha.-amylase to hydrolyze the starch to a D.E. of about 10 to 20. This process is disclosed and claimed in U.S. Pat. No. 3,853,706.
By either process the thinned starch may thereafter be converted to dextrose or dextrose-containing syrups by other enzymes such as glucoamylase. Glucoamylase preparations are produced from certain fungi strains such as those of the genus Aspergillus; for example, Aspergillus phoenicis, Aspergillus niger, Aspergillus awamori, and certain strains from the Rhizopus species and certain Endomyces species. Glucoamylase effects the hydrolysis of starch proceeding from the non-reducing end of the starch molecule to split off single glucose units at the alpha-1,4 linkages or at the alpha-1,6 branch points. Commercial glucoamylase enzyme preparations comprise several enzymes in addition to the predominating glucoamylase; for example, small amounts of proteases, cellulases, .alpha.-amylases, and transglucosidases.
Considerable interest has developed in the use of immobilized enzyme technology for the production of dextrose or dextrose-containing syrups from starch. In this technology, the enzyme, attached to some insoluble support material, may be reused repeatedly, and a more precise control of the reaction is possible. Various procedures have been described for the immobilization of glucoamylase. These include covalently binding an enzyme to an insoluble carrier, adsorption of an enzyme on an insoluble carrier followed by cross-linking of the enzyme to prevent an escape from the carrier, and entrapment of the enzyme within the pores of a porous material. References which review the art of enzyme immobilization, with particular attention to the immobilization of glucoamylase, are given in U.S. Pat. No. 4,011,137.
Several reports have been made of attempts to immobilize glucoamylase on alumina. Usami and Taketomi, Hakko Kyokaiski, 23, 267-9 (1965), reported that various substances including alumina could adsorb "Glucoteem" from solution. However, there was no mention of any further use of the adsorbed material. Solomon and Levin, Biotechnol. Bioeng., 17, 1323-1333 (1975), reported that amyloglucosidase was adsorbed on 4 of the 7 samples of activated alumina they tested. Inactivation of the enzyme composite was observed when it was exposed to a starch hydrolyzate, and the amount of inactivation increased as the substrate concentration increased. When the alumina was treated with a dye prior to adsorption of the enzyme and the mixture was further reacted with glutaraldehyde, the useful life of the immobilized enzyme was increased.
In U.S. Pat. No. 3,850,751, it was disclosed that various enzymes are adsorbed on alumina, titania and zirconia of specified pore size. There was no mention of the binding of glucoamylase to these inorganic supports.
The reported uses of alumina or other inorganic carriers as supports for immobilizing glucoamylase generally requires chemical reaction to cross link the enzyme and/or to attach the enzyme to the carrier. Such chemical treatment destroys much of the enzyme activity and increases the cost of the process. The processes usually operate at temperatures of 50.degree. C. or below. At these temperatures bacterial contamination is frequently a problem, and conversion of the starch hydrolyzate to dextrose is slow. Furthermore, when these enzyme composites are used in plug-flow reactors, the hydrolyzate must be treated at such a slow flow rate that the process is not practical for commercial use.